wallet: Releases dangling files on BerkeleyEnvironment::Close

Summary:

This PR closes db.log and removes .walletlock files when BerkeleyEnvironment is closed.

Backport of core PR15297.

Test Plan:

ninja check check-functional

Reviewers: #bitcoin_abc, nakihito

Reviewed By: nakihito

Differential Revision: https://reviews.bitcoinabc.org/D5085